Well I'm not sure we can give a definitive answer there, it really depends on what you want.

Do you want to support smart phones? Tablets? Just PCs?

If we were making a site and wanted all three, we would normally use a fluid responsive layout. We'd then set the breakpoints such that smart phones saw the responsive-layout (i.e. just one full-width column), tablets saw the grid-layout, and PCs also saw the grid-layout but at a larger width.

I'd make the min-width around 750px, and the max-width whatever was still comfortable to read (somewhere between, say 1000px and 1500px, depending on your layout).